The present invention relates to a method for forming multi-color images by use of an electrophotographic copying apparatus, printers, etc. and more particularly to a method for forming multi-color images with the use of a two-component dry developer.
To form a multi-color image by use of an electrophotographic copying apparatus, printers, etc. there is known a method which develops a first latent image by use of a first toner and a second latent image with a second toner charged to the same polarity as the first toner.
Referring to FIG. 12, an example of the prior art methods will be described:
There is provided a photosensitive image-bearing drum 10a rotatable in the direction of arrow during which charging, a first exposure, a first development, a second exposure, a second development and a toner image transfer are consecutively carried out.
More specifically, around the photosensitive drum 10a (hereinafter called the drum) are disposed a main charger 11a, a first optical system 20a, a first developing unit 13a, a second charger 14a, a second optical system 30a, a second developing unit 16a, a transferring charger 17a, a cleaning device 18a, and an eraser 19a. The development is conducted by steps 1 to 6 shown in FIG. 13 so as to form a two-color image.
Step (1): The surface of the drum 10a is charged at a potential V.sub.O 1 by the main charger 11a, wherein the potential V.sub.O 1 is normally -500 to -1000 V. PA1 Step (2): The drum 10a is exposed to a laser beam 12a from the first optical system 20a, etc. to form a first latent image. PA1 Step (3): The first latent image is subjected to reversal development by the first developing unit 13a at a bias voltage V.sub.B 1 with the use of a two-component developer containing a color toner T.sub.c and a carrier. PA1 Step (4): Where required, the second charger 14a is used to charge the surface of the drum 10a at a potential V.sub.O 2. PA1 Step (5): The charged drum 10a is exposed to a laser beam 15a etc. from the second optical system 30a to form a second latent image. PA1 Step (6): The drum surface having the second latent image is subjected to reversal development by the second developing unit 16a at a bias voltage V.sub.B 2 with the use of a two-component developer containing a black toner T.sub.b and a carrier. PA1 Step 7: The two-color toner image is transferred onto a recording material such as paper by the transferring charger 17a, and fixed thereon by a fixing device (not shown).
This prior art method has disadvantages; one is that some toner T.sub.c used for the first development intrudes into the developer used for the second development. Hereinafter a portion of one toner which mixes with another is called "foreign toner". The more sheets are copied, the more turbid the second developer becomes with a foreign first toner. This problem is particularly remarkable when the developing unit is a magnetic brush type, because the first toner image formed on the drum surface is scraped off by the magnetic brush thereby to cause some of the first toner to mix with the second developer.
In order to avoid the problem of mixed colors, it is important to remove the foreign first toner out of the second developer for which the following methods have been proposed:
1. The polarity of the foreign toner is reversed to effect electrostatic separation. One example is disclosed in U.S. Pat. No. 4,822,702, and the corresponding Japanese Patent Application "Kokai" No. 58-137846. The disclosed method has an arrangement in which the first toner, the second toner and the carrier are arranged in a frictional electricity series so as to effect the reversal of polarity of the foreign first toner. Thus the foreign first toner is removed out of the second developing unit, or adheres to a non-image forming portion of the drum, or else picked up by a roller-type collector.
2. The development thresholds of the first and the second toner are differentiated, which means that the first toner and the second toner require different initiating potentials, and the foreign toner is removed by adhering to the surface of a bias-applied roller. This method is disclosed in Japanese Patent Application "Kokai" No. 58-102251. This is practised by using a first non-magnetic color toner and a second magnetic black toner, and disposing the bias-applied roller in the second developing unit. No magnetic black toner adheres to the roller because of being its magnetic threshold whereas the non-magnetic color toner adheres thereto when the roller is electrically biased. In this way the foreign toner is removed out of the second toner.
The methods described above have the following disadvantages:
In the method (1) the frictional series must be properly regulated, which may limit a range for selecting the kinds of toner components. Another disadvantage is that the performance is susceptible to external conditions such as atmospheric temperature and humidity. A further disadvantage is that the toner scatters and stains the inside of the apparatus, which requires a suction duct or the like for removing it. As a result the apparatus becomes large.
One disadvantage of the method (2) is that a relatively large developing unit is required for accommodating the bias-applied roller, and another disadvantage is that an extra means is required for collecting the foreign toner separated by the bias-applied roller.